1. Field of the Invention
The present invention relates to a time division multiplex (TDM) transmission system particularly suitable for dedicated wired music broadcast, wherein a plurality of digital audio sources or the like having a same sampling frequency and a same quantization bit number are time-divisionally multiplexed, transmitted via transmission line, and demultiplexed.
2. Description of the Prior Art
In conventional dedicated wired music broadcast, various music programs are broadcasted by the frequency multiplex transmission system using a plurality of carrier frequencies.
Recently, digital audio apparatus and PCM digital audio sources are widely used. Since the bandwidth of a wired audio broadcast is not sufficient for a PCM signal to be transmitted, the PCM signal once converted into an analog signal is transmitted. Therefore the advantageous features of a digital source is not positively utilized, but the tone quantity is degraded during transmission.
Presently available PCM digital audio sources each have a different format, transmission rate, sampling frequency, and quantization bit number. Even a digital audio source regenerating apparatus of a same type has a slight difference in its transfer clock frequency, and clocks between apparatus are asynchronous. Therefore, it is difficult to time-divisionally multiplex and transmit various audio signals in a single suitable format.
In a dedicated multi-channel PCM music broadcast, analog signals are once obtained through the D/A conversion of a plurality of digital signal sources, e.g., digital regenerating apparatus such as CD players. The analog signals are again A/D converted to obtain digital signals for the plurality of channels. The digital signals are time-divisionally multiplexed, formatted and transmitted using sampled PCM words.
Signals are once D/A converted and then A/D converted so that a high fidelity PCM audio signal is degraded during such conversion. Further, in a decoding process at a receiving side, an LSI is needed to be newly developed for decoding a formatted low order frame row unit after demultiplexing from a high order to a low order.
BS-PCM audio signals of satellite broadcast (BS), digital audio signals of digital audio apparatus such as CD players and digital audio tape recorder (DAT) have been used as digital audio sources. In this case, analog signals obtained through D/A conversion are modulated and broadcasted.
It is possible to considerably lessen the degradation of tone property if digital signals are modulated and broadcasted without once converting into analog signals.
However, BS-PCM audio signals have a sampling frequency (fs) of 48 KHz, whereas digital audio signals of a commercially available digital audio apparatus such as CD player has a sampling frequency of 44.1 KHz. In addition, the format of each PCM signal is also different.
Therefore, to receive modulated PCM signals, it is necessary for the receiver to be equipped with both processors for BS format PCM signals and CD player format PCM signals. In addition, it is necessary to discriminate between the processors for the corresponding PCM signals. The provision of and discrimination between the processors result in an uneconomical and expensive receiver, and in an obstacle against practical use. It is thus desired to have a measure by which a single signal processor can handle a plurality of PCM signals.
The quantization bit number of a PCM signal of a CD player is 16 same as that of a B-mode BS-PCM audio signal. If PCM signals of a CD player are formatted into the B mode BS-PCM format, both signals can be processed by only a signal processor for the BS-PCM signals.
Although the quantization bit number is 16 for both PCM signals, the sampling frequency (fs) is 48 KHz for BS-PCM audio signals and 44.1 KHz for PCM audio signals, and hence the transmission rate is 2.048 Mb/s and 1.8816 Mb/s, respectively.
Therefore, clocks suitable for each transmission rate become necessary for demodulating PCM audio signals. Thus, a clock regenerating circuit for each transmission rate becomes necessary.
As discrimination means for transmission rates, a PLL circuit having a broad capture range has been used heretofore. Based on an input voltage to the VCO of the PLL circuit, a transmission rate discrimination signal is generated. A most suitable PLL circuit among a plurality of PLL circuits having a narrow capture range is selected to regenerate a clock in accordance with the transmission rate.
However, the above conventional transmission rate discrimination means requires a PLL circuit having a broad capture range for reducing the number of signal processors for generating a transmission rate discrimination signal, and a plurality of PLL circuits having a narrow capture range each selected by the transmission rate discrimination signal. Further, clocks corresponding to the transmission rate are required to be regenerated by the selected PLL circuit.